1. Field of the Invention
The present invention relates to a wide-band loop antenna and, more particularly, is directed to a wide-band loop antenna which is preferably applied to a glass window antenna of a motor vehicle.
2. Description of the Prior Art
A glass window antenna attached on a widow glass of a motor vehicle is well-known. The antenna comprises a conductor arranged on the window glass to feed a reception signal to an AM radio receiver, an FM radio receiver or a TV receiver mounted in a automobile (refer to for example, Japanese patent publication No. 33951/1975 and Japanese patent laid-open application No. 44541/1977). The antenna conductor is usually arranged on a rear window glass having relatively large area. The central portion of the rear window however is necessary to ensure rear view of a driver. It is therefore undesirable to attach the antenna conductor on the central portion. Further, defogging heater conductors are usually provided on the central portion so that the antenna conductor can not be attached on that portion. The antenna conductor therefore must be arranged in a narrow blank area upper or lower the defogging heater conductors.
It is difficult on that narrow blank area to wire the antenna conductor complexly or to add an array of parasitic elements as the Yagi-Uda antenna for obtaining a wide-band characteristic. It is therefore difficult to provide to a conventional window glass antenna a good frequency characteistic over a whole range of FM broadcast band.
Mean while, a loop antenna is known as one having relatively wide-band characteristic though it is considerably simple. The loop antenna is advantageous as it can be arranged along the periphery of a glass having heater conductors on the central area thereof.
The loop antenna however has not so wide-band characteristic to cover a considerable wide range for example from an FM broadcast band to a TV broadcast band. Two or more antennas must be provided on a glass window to receive reception power and feed to receivers such as an FM radio receiver and a TV receiver when both of them are mounted in an automobile. In addition, feeder cables for feeding reception power to the receivers must be attached to each of the antennas. The feeder cables make the peripheries of the window complicated and.
The loop antenna furthermore has a defect in that it has a high output impedance as large as 300 [.OMEGA.] for an oblong rectangle loop antenna with a power feed point at a longer side when used in FM broadcast reception. Sensitivity is lowered due to mismatching of impedance when a coaxial feeder cable of 50 [.OMEGA.] is directly connected to the feed terminal. A specialized matching circuit must be provided between the loop antenna and the feeder cable to match the impedance in the prior art.